In a conventional vehicular electric system, a first motor driver device drives a vehicle travel motor and a second motor driver device drives a vehicular air-conditioner motor.
In the vehicular electric system, a first capacitor is connected between two power input terminals of the first motor driver device and a second capacitor is connected between two power input terminals of the second motor driver device thereby to suppress variations in voltages supplied from a DC (direct current) power source to the first and the second motor driver devices.
The output drive power of the travel motor is greater than that of the air-conditioner motor. The input electric power supplied from the DC power source to the first motor driver device is hence greater than that supplied from the DC power source to the second motor driver device. The capacitance of the second capacitor is thus less than that of the first capacitor.
According to JP H02-223301A (patent document 1), a resistor is connected in parallel to a coil between an inverter circuit and the DC power source to suppress resonance from arising between the inverter circuit and the DC power source.
According to JP H10-311646A (patent document 2), a plurality of coils is provided in parallel between an inverter circuit and a DC power source and one of the coils is selectively connected by a change-over switch between the inverter circuit and the DC power source to suppress a harmonic current from flowing between the inverter circuit and the DC power source.
According to the vehicular electric system, if the capacitance of the second capacitor is less than that of the first capacitor as described above, a ripple current flows from the first motor driver device to the second capacitor side when the first motor driver device drives the travel motor. The voltage between the two power input terminals of the second motor driver device responsively varies.
It is possible to connect a coil between the DC power source and the second motor driver device to suppress the ripple current from flowing from the first motor driver device to the second capacitor side. Even in this case, the ripple current cannot be suppressed completely and hence flows from the first motor driver device to the second capacitor through the coil.
The first and the second capacitors form together with the coil a π-filter circuit having a specific resonance frequency. In case that the switching frequency, at which the first motor driver device drives the travel motor, and the resonance frequency of the π-filter circuit overlap, the ripple current triggers a resonance in the π-filter circuit. As a result, a large current flows to the first and the second capacitors because of the resonance in the π-filter circuit and affects the capacitors adversely.
It is possible to connect a resistor in parallel to a coil between the DC power source and the second motor driver device as disclosed in the patent document 1. The resistor however causes loss in power supplied from the DC power source to the second motor driver device.
It is also possible to avoid an overlap of the resonance frequency of the π-filter circuit and the switching frequency by selectively connecting one of the plurality of coils connected in parallel between the second motor driver device and the DC power source by the change-over switch. Addition of the coils and the change-over switch however causes an increase in size.